In present day seismic exploration there are several methods for acquiring data in a marine environment. The most common method is to use a marine vessel to tow a line of seismic acoustic pulse detectors behind a line of acoustic pulse generators. Unfortunately, this type of arrangement does not permit extremely long spacing between the line of acoustic pulse generators and acoustic pulse detectors. In a system where a line of acoustic pulse detectors are towed behind the. acoustic pulse generator, reflection data is obtained. Reflection data is that data based upon returning acoustic waves that are reflected back from a subsurface interface or change in density. By extending the spacing between seismic pulse generators and seismic pulse detectors, refraction data may also be obtained. Refraction data is that data based upon acoustic waves that are returning back from the source side of an interface after traveling along the subsurface interface.
A method for acquiring marine seismic data which permits extended spacing between seismic pulse generators and seismic pulse detectors is the use of an on-bottom seismic cable. By extending the spacing between a seismic pulse source or generator and seismic pulse receivers, refraction and reflection data may be obtained.
An on-bottom seismic cable is similar to a streamer cable of seismic pulse detectors such as that towed behind a marine vessel. A streamer cable comprising a plurality of hydrophone groups spaced along its length can be used as an on-bottom cable, with the proper weight added to remove buoyancy. This assures that the streamer will sink and remain fairly stationary despite the ocean-bottom currents. More detailed information concerning the construction of an ocean bottom seismic cable can be found by referring to copending patent applications Ser. Nos. 579,042, 579,042 and 579,043, titled "Strain Member Chassis", "On Bottom Cable Termination" and "Jacketed Cable Section", respectively, all assigned to the present assignee.
Although an ocean bottom cable has a marker buoy at each end, the length of the cable is several miles and much irregular terrain lies between the buoys. As such, the cable will have a depth variance and a variance from a straight line connecting the two marker buoys. The depth of the ocean bottom cable can be indicated by depth detectors which may be spaced along the length of the cable. The actual location of the acoustic pulse detectors is unknown because the deviation from the straight line connecting the buoy locations is not determined and can be as great as several hundred meters.